1. Field of the Invention
The invention is in the field of synthesis gas and methanol production.
2. Description of the Prior Art
Hydrocarbonaceous materials, including pertroleum oil and natural gas, coal and the like--i.e., fossil energy--are becoming much more expensive and of lower availability. As long as petroleum oil was plentifully available transportation and other liquid fuels could readily be obtained by well-known and relatively inexpensive refining processes.
But with increasing scarcity such refined liquid fuels will no longer be sufficient. Much research and development effort has been expended to supplement such fuels. It is, for example, well known that "synthesis gas"--a mixture of mainly carbon monoxide and hydrogen--may be converted particularly over appropriate catalyst, to a wide variety of increasingly valuable products, including hydrogen, carbon monoxide, ammonia, and hydrocarbonaceous and oxyhydrocarbonaceous liquids. Possible liquid products include methanol, ethanol, higher alcohols, ethylene glycol, and liquid hydrocarbons. For the most part they can either be used directly or further converted to chemical products or to liquid fuels.
This synthesis gas may be--and has been--derived from any of the hydrocarbonaceous raw materials by reacting these with steam and/or oxygen at relatively high temperatures. However, the great disadvantage of such prior processes, particularly those which convert one fuel into another fuel, is that the thermal energy efficiency is relatively low: one ends up with substantially less fuel heating value than one started with. The state-of-the-art processes are thus wasteful of increasingly scarce and expensive fossil energy.
Thus, in a recent, carefully engineered, state-of-the-art comparison by the broadly experienced Lurgi Company ["Gasoline Production from Natural Gas or Coal," E. Supp, 1980, Table II], the following thermal efficiencies (based on the lower heating values of starting materials and end products--with by-product electricity calculated back to equivalent heat on a reasonable basis) are shown to be economically attainable.
______________________________________ Thermal Efficiencies, % Case: I II III IV Raw Material: Products Natural Gas Coal ______________________________________ Methanol 61.8 66.5 48.6 Methanol + 62.9 Electricity Methanol + 58.4 Methane Fuel Gasoline 56.1 64.5 46.2 Gasoline + 57.6 Electricity Gasoline + 55.8 Methane Fuel ______________________________________
In Case I, conventional steam reforming of natural gas to produce synthesis gas, which is then compressed and catalytically converted to methanol, is utilized. For the production of gasoline, the methanol is catalytically converted using the Mobil Corporation methanol-to-gasoline (MTG) process.
Case II is similar except that oxygen is added in a second stage of (autothermal) reforming. Case III autothermally reforms coal with steam and oxygen; and, in Case IV, the purge gas is catalytically methanated to form a synthetic natural gas. In all cases fuel necessary for all utilities required in the process is furnished and included in the calculations.
Several significant conclusions are apparent from the values given above. First, the above approaches to the conversion of coal to more useful liquid fuels has a higher thermal efficiency than other methods: Fischer-Tropsch (about 40%) and (improved) coal hydrogenation (about 54%). Second, natural gas is much more efficient as a raw material than coal. In terms of energy losses--in comparing the highest values given, using coal loses over 24% more energy than in using natural gas. Conservation of overall energy considerations therefore favor using natural gas for this purpose--provided of course that there are other uses for coal in which its relative thermal efficiency is higher than in the above use. (Such a use is indeed direct generation of electricity in steam-electric stations, where coal should displace natural gas. For their thermal efficiencies are about equal).
In addition, the overall capital investment cost for the use of coal to produce liquid fuels is about three times that for use of natural gas. In consequence of both of these factors--in which coal wastes both scarce energy and scarce capital--natural gas cost would have to rise to about six times the cost of coal before coal becomes economically competitive for these purposes. Fortunately, it has been estimated that sufficient natural gas has been, or is still to be, found in the world so that it is nearly as plentiful as coal is, so that, from an economic standpoint for many decades hence, natural gas will be favored for these purposes.
Next, it is seen that an excess of relatively low temperature by-product heat is produced, which can be converted into electricity, in Case I, that of conventional steam reforming. However, such does not improve the overall thermal efficiency greatly. A much better practice is to go to Case II. This, however, would require the added investment of an air-separation oxygen plant and compressor, which has seldom been economically justified or practiced.
Finally, it is seen that even with the various combinations thermal efficiencies are still quite low and represent a very substantial loss of energy.